Abstract

Glucocorticoids are important physiological and pharmacological regulators of epidermal proliferation, differentiation and skin tumorigenesis. The glucocorticoid effects are mediated via the glucocorticoid receptor (GR), a transcription factor that regulates genes expression by DNA-binding dependent transactivation, and DNA-binding independent transrepression through negative interaction with other transcription factors. Our previous studies demonstrated that keratin5.GR transgenic animals are resistant to skin carcinogenesis. We also found that GR diminishes the number of follicular epithelial stem cells (SC), reduces their proliferative and survival potential and affects the expression of follicular SC “signature” genes. To further explore the tumor suppressor role of GR in skin carcinogenesis we generated skin-specific GR knockout animals by breeding of GRflox/flox animals with Keratin5.Cre recombinase animals in C57Bl genetic background. Skin-specific GR KO (Cre/GRfl/fl) mice do not have gross skin or hair phenotype. However, the detailed analysis of keratinocyte markers revealed decreased expression of some intermediate/late markers of differentiation including Keratin 10 and loricrin. We also found that Cre/GRfl/fl mice are more sensitive to the effects of UV irradiation and tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). The ongoing DMBA/TPA skin carcinogenesis experiment showed that Cre/GRfl/fl mice are more prone to skin carcinogenesis: they have higher tumor incidence and tumor multiplicity compared to GRfl/fl littermates. The analysis of global effect of the GR overexpression and GR silencing on gene expression in follicular epithelial SC, basal keratinocytes, and mouse skin tumors 1) revealed important GR targets in keratinocytes including STAT3 and REDD1 - the major regulator of mTOR; 2) provided an unexpected evidence that gene transrepression by GR plays critical role in the maintenance of SC and in inhibition of skin carcinogenesis. It is well understood that therapeutic effects of glucocorticoids are mediated by GR transreprssion, while many adverse metabolic effects are mediated by GR transactivation. Thus, our findings are important in light of drug discovery programs focused on the development of selective GR acivators (SEGRA) that preferentially induce GR transrepression. Some of SEGRA entered preclinical studies or clinical trials as anti-inflammatory GR ligands with reduced side effects. Our results suggest that SEGRA could be also used for skin cancer prevention/treatment.